1. Field of the Invention
The present invention relates to a control system and control method for controlling an information rate (of data signal) in a mobile communication system and a radio network controller preferably used for the system and method.
Particularly, the present invention relates to a control system and control method for controlling an information rate through a downlink addressed to a mobile station in a mobile packet communication system and a radio network controller.
2. Description of the Related Art
A conventional mobile packet communication system addresses time-division multiplexed data signals to a plurality of users (mobile stations) on a single radio channel (hereafter referred to as a channel) and transmits the data signals to the users from a viewpoint of the efficiency of frequency utilization, delay tolerance in packet communication, and the saving of code resources in downlink.
FIG. 1 shows a channel configuration for a conventional packet communication system specified in 3GPP (Third Generation Partnership Project).
The channel configuration shown in FIG. 1 uses a downlink shared channel (DSCH) for time-division-multiplexing and transmitting data signals addressed to mobile stations 401 and 402 through a downlink.
The channel configuration shown in FIG. 1 uses an associated dedicated physical channel (A-DPCH) for transmitting control signals addressed to the mobile stations 401 and 402 through a downlink.
Moreover, the channel configuration shown in FIG. 1 uses a dedicated physical channel (DPCH) for transmitting both data signals and control signals addressed to the mobile stations 401 and 402 in uplink because the possibility of exhaustion of code resources is very low in uplink and the necessity for using a shared channel such as the DSCH is low.
In this case, A-DPCH-1 and A-DPCH-2 are dedicated channels for a base station 30 to transmit a control signal to the mobile stations 401 and 402 respectively, which are always transmitted through code division multiplexing using codes different for each of the mobile stations 401 and 402.
Moreover, the DSCH is a shared channel for the base station 30 to transmit data signals to the mobile stations 401 and 402 and is always transmitted through code-division multiplexing for changing the transmission timing for each of the mobile stations 401 and 402 using the same code.
In this case, to transmit data signals from the network (base station 30) to the mobile station 401 or 402 by using the DSCH, the message “DSCH is transmitted” is previously notified to the mobile station 401 or 402 through the radio frame of the A-DPCH before the radio frame of the DSCH.
Then, the mobile station 401 or 402 receiving the above notice starts receiving the DSCH only when it is determined that the DSCH is coming.
By using the above system, the base station 30 can freely change the mobile stations 401 and 402 for transmitting data signals by using the DSCH.
In FIG. 1, the A-DPCH-1 and A-DPCH-2 and the DSCH are set as downlinks. However, the DSCH is not always set but is set only when the above notice is received through the A-DPCH-1 or A-DPCH-2.
Because a DSCH is discretely transmitted on a time base, it is impossible to apply closed-loop transmission power control to the DSCH. Therefore, closed-loop transmission power control is applied to a temporally-continuous downlink A-DPCH or uplink DPCH, the transmission power of the DSCH is controlled so as to interlock with the transmission power of the A-DPCH with a predetermined offset which is previously decided by a telecommunication operator.
For example, when the transmission power of the A-DPCH-1 addressed to the mobile station 401 is 20 dBm at the time t and the DSCH is transmitted to the mobile station 401 at the time t, the transmission power of the DSCH addressed to the mobile station 401 at the time t becomes 30 dBm, if the above offset is equal to 10 dBm.
Moreover, for example, when the transmission power of the A-DPCH-2 addressed to the mobile station 402 is 22 dBm at the time t+1, the transmission power of the DSCH addressed to the mobile station 402 at the time t+1 similarly becomes 32 dBm.
Furthermore, in the case of a conventional mobile packet system specified in 3GPP, the information rate of the DSCH can be changed at every transmission time interval (TTI) at each of mobile stations 401 and 402.
However, it is unknown what can be used as a trigger to change an information rate of the DSCH at present, and an information rate set at the start of communication is not changed after the start of the communication.
However, there is a problem in that the operation of the above DSCH at a fixed information rate cannot follow a change of radio propagation conditions and thereby, the communication quality of the DSCH may be deteriorated.
That is, when the communication quality of the DSCH deteriorates, it is observed as a phenomenon such as an increased number of transmission errors (e.g. block errors) or increased transmission delay when viewed from mobile stations 401 and 402. To provide services, it is preferable to minimize the deterioration of communication quality.
On the other hand, if the communication quality of the DSCH is too high, an increased number of transmission errors (such as block errors) or increased transmission delay is not observed when viewed from the mobile station 401 and thereby, a preferable communication environment is obtained.
However, because it is impossible to ignore the interference to the other mobile station 402 of its Own cell or the mobile station 40 of the other cell, the above excess of quality is not preferable.
In general, it is a problem for a mobile packet communication system to maintain communication quality, decrease communication delay (increase an information rate), and simultaneously connect with numerous mobile stations 40 (accommodate more users). In this case, it is known that an information rate is proportional to a transmission power.
That is, it is known that when a high information rate is required, a large transmission power is needed and when communication is performed at a low information rate, only a small transmission power is necessary.